ts-get-interp causes runtime error when input time is at/near start of time series

Using ts-get-interp with a time equal to, or soon after, the first time in the time series object raises a runtime error in to-report __col-of-row [columns row col-name]; the error is "ITEM expected input to be a string or list but got NOBODY instead."

Tracing this back through __interp, I am guessing the problem has to do with ts-get-before-and-after sometimes returning NOBODY...

The documentation for ts-get-interp needs to say what happens when you try to interpolate using a time not between the beginning and end of the time-series.

Reproduce the boundary cases for ts-get-interp, then trace __interp and ts-get-before-and-after to confirm how NOBODY reaches __col-of-row. Review the extension documentation for interpolation behavior outside the time-series range; done means the runtime behavior is understood and the documented boundary behavior matches it.